The proposed research will result in the development of simplified techniques for characterizing the x-ray energy spectra from machines used in radiation therapy in the energy range from 0.2 MV to 10 MV. Once developed, the techniques will be extended and modified as necessary to make them useful also for diagnostic radiology equipment. The characterization of energy spectra is important for complete quality assurance programs in both diagnostic and therapeutic radiology. The simplified techniques to be developed in this project will be an important tool for such quality assurance. We will use an attenuation measurement technique and analyze the resulting x-ray transmission data by two different methods which are extensions and improvements of previously proposed methods. Analysis of the transmission data from a particular machine will result in a reconstruction of a representation of the x-ray energy spectrum for that machine. We will use computer simulations to test various attenuating materials and modifications of the measurement technique to determine the most appropriate approach to actual measurements for a particular machine. Attenuation measurements will be made with combinations of detectors and attenuators which result in transmission curves that are amenable to analysis using our techniques. We will compare our reconstructed spectral representations with measured and calculated spectra from similar machines and geometries and specify the measurement and analytical techniques most appropriate for determining a characterization of the energy spectrum. These techniques are likely to differ with machine depending on its energy range and spectrum. Finally, these techniques will be extended to diagnostic x-ray machines and modified to account for the characteristic x-rays present in more lightly filtered beams.